Stamina mode only at 5 hours (100% charge) topic

I need your input Z1 users.

Ok I just bought an Xperia Z1. Everything seems to work fine except I can not get the Stamina mode to increase to 6 days and more. I've seen people online (forums/reviews)getting it up to 15 - 20 days!

I basically I turned off everything. Stamina mode @ 100% charge is 5 days & 11 hours. If I turn the Stamina mode off.. Just 2 days and several hours.

Did I get a faulty battery? Any other settings I should know of? I haven't downloaded anything yet from the PlayStore.

Model : C6903
Android Ver: 4.3
Firmware : 14.2.A.0.290

TIA!